An optical element such as a wavelength multi/demultiplexing element is used for optical communication and optical interconnection. Silicon (Si) photonics has been recently drawing attention as a promising technology for large-capacity interconnection, and an improvement of transmission capacity per one photonic wire by wavelength division multiplexing (WDM) in a Si chip is expected. A wavelength multi/demultiplexer of Si photonic wire waveguide type, for example, is used as a multiplexer which multiplexes WDM optical signals, and a demultiplexer which demultiplexes a WDM optical signals for transmission/reception of WDM optical signals in the Si chip. As an example of such a multi/demultiplexer, there is a delayed interferometric element, and various studies regarding delayed interferometric elements have been conducted.
For example, it is reported that a flatness of multi/demultiplexed spectrum is improved by loading a ring waveguide coupled to a waveguide between a pair of optical couplers. However, even if the ring waveguide is used, it is difficult for a conventional delayed interferometric element to obtain a sufficient flatness without conducting a temperature control.
Patent Literature 1: Japanese Laid-Open Patent Publication No. 2013-186358
Patent Literature 2: Japanese Laid-Open Patent Publication No. 2013-205626
Non-Patent Literature 1: D. W. Kim, A. Barkai, R. Jones, N. Elek, H. Nguyen, and A. Liu, “Silicon-on-insulator eight-channel optical multiplexer based on a cascade of asymmetric Mach-Zehnder interferometers,” Optics Letters 33(5), 530-532 (2008)
Non-Patent Literature 2: F. Horst, “Silicon integrated waveguide devices for filtering and wavelength demultiplexing,” in Proceedings of 2010 Optical Fiber Communication Conference, OWJ3
Non-Patent Literature 3: J. Song, Q. Fang, S. H. Tao, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Passive ring-assisted Mach-Zehnder interleaver on silicon-on-insulator,” Optics Express 16(12), 8359-8365 (2008)
Non-Patent Literature 4: S.-H. Jeong, S. Tanaka, T. Akiyama, S. Sekiguchi, Y. Tanaka, and K. Morito, “Flat-topped and low loss silicon-nanowire-type optical MUX/DeMUX employing multi-stage microring resonator assisted delayed Mach-Zehnder interferometers,” Optics Express 20(23), 26000-26011 (2012)